Bolometers or bolometric detectors detect an increase in temperature when a thermally isolated membrane is heated by absorption of electromagnetic radiation. The change in temperature is typically sensed with a thermistor. If the thermistor is a superconducting metal biased on the sharp resistive transition, the resistance increases in response to the temperature increase. If the thermistor is made of a semiconductor, the resistance decreases in response to the temperature increase. At the operating temperatures of interest—in addition to Neutron-Transmutation-Doped (NTD) Ge, NbSi thin film thermometers, and metallic magnetic calorimeters (MMC)—superconducting transition edge sensors (TES), microwave kinetic inductance detectors (MKID), etc. can also be used to sense a change in absorbed power by monitoring a change in the device operating point through an appropriate monitoring circuit network.
Bulk materials, such as, for example, silicon and silicon nitride employing very long (1-2 mm) and narrow (˜1 μm) bridges can be used for thermal isolation of a membrane in space-borne sub-mm/far-IR instruments, however, this approach can have multiple undesirable effects and disadvantages in practice. Focal plane architecture and scalability for use in large-format arrays as well as fabrication yield can be compromised. Furthermore, susceptibility of a detector to low-frequency mechanical vibrations (e.g., parasitic microphonic response) can be increased and ultimately degrade the sensor performance. As a result, the detector output may be modulated because of its low stiffness and coupling to other components in the focal plane architecture. Also, the heat capacity, response time and noise of the detector may be undesirable due to the thermal mass of the bridges. Still further, shielding against stray light or out-of-band radiation, which can couple through free-space absorption or directly into the readout circuitry, is challenging for pixel sizes approaching or greater than the shortest detected wavelength. Even further, the conductance of long bridges can be particularly sensitive to surface roughness and deposited organic residues introduced in device fabrication.
Exploration of narrow-band periodic one-dimensional phononic structures reveals the presence of higher order spectral leaks or, in a disordered limiting case, such structures can require relatively long structures with defects along the length to achieve the desired spectral response. From a fabrication and performance standpoint, such structures are problematic. Such structures can result in a reduction of the thermal conductance by increasing the porosity, however, this can require an extremely complex and high uniformity process for device repeatability given the exponential dependence on the underlying parameters.
Although periodic filter structures have been explored in two- and three-dimensions for two-transverse and one-longitudinal phonon modes, it would be desirable to have a broadband phononic filter that overcomes the above limitations in design and fabrication for controlling thermal and phonon transport as well as use of a fabrication method that would result in devices of the required length of scale while achieving lower conductance than possible in the ballistic limit.